When a fluid passes by an object or obstruction, oscillations can occur. Examples of these oscillations in nature include the whistling caused by wind blowing by the branches of trees, the swirls produced downstream of a rock in a rapidly flowing river, and the waving of a flag in the wind. Note that in all of these examples, when the flow is slowed, the oscillations stop. That is, the whistling stops when the wind dies down, the water flows calmly around the rock when the river is not flowing rapidly, and the flag does not wave in a mild breeze.
Vortex flow meters operate under the same principle. Vortex flow meters are usually installed in-line within a process-fluid flow conduit to measure the flow rate of a fluid. Vortex flow meters are based on a fluid instability known as vortex shedding, which occurs when a fluid flows around an obstacle, usually called a bluff body, inside a pipe system. Since the fluid flow does not adhere to the obstacle contour due to the boundary layer effect, flow separation occurs behind the bluff body thereby forming vortices where the pressure is low behind the body. These vortices alternate downstream to generate flow instability called Karman vortex street. These vortices are shed alternately from one side to the other of the bluff body at a defined frequency that is proportional to the flow rate in the pipe. Similar to the waving flag, the frequency of the vortex shedding increases with increasing fluid flow. In these types of meters, the differential pressure resulting from the vortices being formed and shed is sensed by sensors that measure a frequency proportional to the vortex shedding mechanism and a transmitter generates a flow measurement signal based on the measured frequency.
FIG. 1 shows an isometric view of a vortex flow meter 10 known heretofore. Process fluid 12 enters vortex flow meter 10 and flows past bluff body 20, which in turn causes the formations of alternating vortices 22. FIG. 2 shows a top view of the same vortex flow meter 10. FIG. 3 shows a computational flow simulation of the Karman vortex street. As can be seen, the vortices are generated behind the bluff body in alternate form. Additionally, for a defined geometry of the bluff body, the Karman street vortex frequency is proportional to flow rate.
Vortex flow meters measure the velocity of liquids, gases and vapors in pipes, such as water, cryogenic liquids, boiler feed water, hydrocarbons, chemicals, air, nitrogen, industrial gases, and steam. However, a known shortfall for vortex flow meters exists in applications where flow measurement is required near the bottom of the vortex flow meter's range because the sensors within the vortex flow meters turn off at low flow rates. The velocity at which the sensors turn off is typically 0.3 m/sec (1 ft/sec) for liquids; however, for gases/vapors, the cut off is much higher due to the relatively low density of the gas/vapor required to operate the sensing system. Therefore, current flow meters do not allow for low flow rates for gases.
Generally speaking, as the size of the bluff body or meter housing decreases, the vortex frequency increases. Furthermore, as the frequency increases, the pressure sensor signal strength is reduced. Therefore, reduced housing sizes results in reduced pressure sensor signal strengths. Naturally, this thereby limits the acceptable range of meter sizes.
An additional problem affecting the accuracy of vortex flow meters is noise. Noise generated by pumps, valves, upstream flow restrictions, compressors, and the like can cause the sensor to read higher output signals, thereby resulting in an inaccurate flow rate reading. The effect of process noise on the reading can be reduced when the sensor's signal-to-noise ratio is at a maximum value. With liquids, the noise problem is not as big of a problem; however, steam and gas fluids generate relatively lower sensor signal strengths, which can be difficult to differentiate from process noise, particularly at low flow rates.
Filters have been use to help eliminate process noise; however, these filters raise the threshold value of the low flow cut off and lead to further misreadings. The result is that the more filtering used to eliminate process noise, the less the net range of the flow meter.
Vortex frequencies typically range from one to thousands of pulses per second, depending upon the flow velocity, the character of the process fluid, and the size of the meter. In gas service, for example, frequencies tend to be about 10 times higher than in liquid applications. (Vortex meters have flow limits based upon the flowing density multiplied by the squared value of the flowing velocity. Therefore with gas applications (with lower density values than liquids), the maximum velocity and consequent frequency limit is much higher than liquid applications.
Therefore, it would be advantageous to have a flow meter that was more accurate than conventional vortex flow meters, particularly for fluids that suffer from problems associated with low flow rates or low densities.